Spitzer deep images have revealed the existence of mid-IR sources that are faint, or even undetected, in some of the deepest HST maps, with colours H-[4.5]>4 (AB). These extremely red colours can be produced by the combined effect of high redshifts and significant dust attenuations. In the first part of my talk, I will present our recent study of such sources using the deep Spitzer SEDS and HST CANDELS images of the UDS field. I will argue that most of these sources are dust-obscured, massive galaxies at z>3. In the second part of the talk, I will show the results of a pilot project to follow a subset of these galaxies up at sub-/millimetre wavelengths. Finally, I will do a combined analysis of the multi-wavelength information for these objects, and discuss the implications of our findings.